The present invention relates in general to power factor correction for minimizing the adverse effects of an inductive load on an alternating current power supply, and more particularly, to a static direct current motor drive of improved power factor.
It is well known that an output regulated AC to DC converter can impair the factor of the AC power supply drawn therefrom. This is especially true of a DC motor drive which is required to be stopped and started repeatedly during operation and that must be forced to reverse its speed quite often. These are common requirements with hoists and cranes. Therefore, it is desirable to have a DC motor drive in which power factor correction is automatically effected during such varying operative conditions.
One particularly desirable type of motor drive is the dual converter because it lends itself to direct current motor control in either direction, and regenerative braking is inherently provided with equal ease for each direction of rotation. The dual converter has also the advantages of any static motor drives which include power efficiency, capability for solid state modular assembly, superior maintainability a high speed in control and flexibility due to the use of a control logic. The dual converter has been described in chapter 5, pages 111-144 of the book by B. R. Pelly, entitled "Thyristor Phase-Controlled Converters and Cycloconverters" edited by John Wiley & Sons in 1971.
As explained on page 127 of this book by B. R. Pelly, when reversal of load current is required, the dual converter can be operated with a circulating current rather than in a circulating current-free mode. This is achieved by connecting a current-limiting reactor between the DC terminals of the two converters forming the dual converter and by regulating the firing angles of the individual converters in such a way that a controlled amount of current is allowed to circulate between them. By so doing, the two individual converters can be in continuous conduction over the whole control range.
In a quite different context, it has been shown by B. R. Pelly and L. Gyugyi in U.S. Pat. No. 4,013,937, issued Mar. 22, 1977 that a naturally commutated static frequency changer coupled to an alternating current power supply may become a controllable static reactive power generator under proper control of a circulating current between opposite banks of static switches.
It is also known from U.S. Pat. No. 4,001,670 of L. Gyugyi, John Rosa and E. J. Stacey, issued Jan. 4, 1977 to connect a fixed capacitor at the input of an unrestricted or of a naturally commutated frequency changer to extend the reactive power range of control available against the alternating current power supply.
Converters which operate over a wide range of firing angles (such as those supplying armature power in variable speed DC motor drives) draw widely varying lagging reactive power from the AC lines. In certain applications (such as large shovels operating from long transmission cables), good compensation of the varying reactive power is required. Because of the wide variation of reactive power, compensation with permanently connected capacitor is not feasible. Switching of capacitors is generally not practical because of the arduous duty of the circuit breakers which are required to operate rather frequently.
An object of the present invention is to provide an improved AC supplied direct current motor drive which can be maintained automatically at a power factor of substantially unity.
Another object of the present invention is to provide a dual converter mounted as a static direct current motor drive between an alternating current power supply and at least one direct current motor, in which the dual converter is automatically controlled to compensate for the inductive load.
Still another object of the present invention is an AC powered hoist system of improved characteristics.